In mammals, Acetyl-CoA carboxylase (ACC) exists as two isozymes. ACC1 is generally expressed in all tissues but its expression is higher in lipogenic tissues such as liver and adipose tissue. ACC2 is highly expressed in muscle tissue and to a lesser extent in liver tissue.
ACC has a central role in lipid metabolism. Malonyl-CoA, the product of the ACC-catalyzed reaction, inhibits mitochondrial fatty acid oxidation through direct inhibition of carnitine palmitoyltransferase 1 (CPT-1), and so controls the switch between carbohydrate and fatty acid oxidative utilization in liver and skeletal muscle. Malonyl-CoA is also a key intermediate in the de novo synthesis of lipids. When metabolic fuel is low, ACC is turned off by phosphorylation and the consequential reduction of levels of malonyl-CoA leads to generation of ATP by increasing fatty acid oxidation and decreasing consumption of ATP for fatty acid synthesis. Thus, in addition to inhibition of fatty acid synthesis, reduction in malonyl-CoA levels through ACC inhibition may provide a mechanism for increasing fatty acid utilization.
By decreasing de novo fatty acid synthesis and increasing fatty acid oxidation in liver, chronic administration of an ACC inhibitor may deplete liver triglyceride and other pathological lipid species, leading to improved liver function and hepatic insulin sensitivity. One might also expect a reduction in the secretion of triglyceride rich lipoprotein (VLDL), so reducing the risk of atherosclerosis.
Therefore, a well-tolerated agent that effectively and simultaneously treats the multiple risk factors associated with metabolic syndrome would have a significant impact on the prevention and treatment of the cardiovascular disease associated with obesity, hypertension, diabetes and atherosclerosis.
Metabolic syndrome (a.k.a. insulin resistance syndrome, syndrome X) is a common clinical disorder that is defined as the presence of increased insulin concentrations in association with other disorders including visceral obesity, hyperlipidemia and dyslipidemia, hyperglycemia, hypertension, and sometimes hyperuricemia and renal dysfunction.
Metabolic syndrome is considered by many as a common basic defect for type 2 diabetes, android obesity, dyslipidemia, and hypertension, leading to a clustering of these diseases. This syndrome has particular significance since it has been shown to be an antecedent of both type-2 diabetes and atherosclerosis, with cardiovascular events accounting for the majority of deaths in both populations.
It is estimated that more than 100 million people in the U.S. alone suffer from some form of metabolic syndrome.
Type 2 diabetes is a severe and prevalent disease in the Western world that affects roughly 13 million persons in the U.S., along with 5 million presumed to have undiagnosed type 2 diabetes and another 14 million with impaired glucose tolerance.
Projections indicate that the incidence of type 2 diabetes will increase to over 25 million by 2010 in the U.S., and to over 300 million worldwide by 2025. The annual direct medical cost associated with type 2 diabetes in the United States is significant, primarily due to the costs of hyperglycemia-related complications, such as retinopathy, nephropathy, peripheral neuropathy, and cardiovascular, peripheral vascular and cerebrovascular disease. Although the causes of type 2 diabetes have not yet been identified, it is well established that it is a polygenic disease characterized by multiple defects in insulin action in muscle, adipose, and liver, and defects in pancreatic insulin secretion. However, the relative importance of each of these defects to the etiology of type 2 diabetes is not clear.
In spite of the early discovery of insulin and its subsequent widespread use in the treatment of diabetes, and the later discovery of and use of sulfonylureas, biguanides and thiazolidenediones, such as troglitazone, rosiglitazone or pioglitazone, as oral hypoglycemic agents, the treatments for diabetes are currently believed to be inadequate. The use of insulin typically requires multiple daily doses. Determination of the proper dosage of insulin requires frequent estimations of the sugar in urine or blood. The administration of an excess dose of insulin causes hypoglycemia, with effects ranging from mild abnormalities in blood glucose to coma, or even death. Treatment of type 2 diabetes usually consists of a combination of diet, exercise, oral hypoglycemic agents, e.g., thiazolidenediones, and in more severe cases, insulin. However, the clinically available hypoglycemic agents can have side effects that limit their use. Moreover as few as 26% of patients with type 2 diabetes achieve target control using current therapies.
Obesity is a major health risk that leads to increased mortality and incidence of Type 2 diabetes mellitus, hypertension and dyslipidemia. It is the second leading cause of preventable death in the United States, and contributes to >300,000 deaths per year. In the U.S., more than 50% of the adult population is overweight, and almost ¼ of the population is considered to be obese (BMI greater than or equal to 30). Furthermore, the prevalence of obesity in the United States has increased by about 50% in the past 10 years. The prevalence of obesity in adults is 10%-25% in most countries of Western Europe. While the vast majority of obesity occurs in the industrialized world, particularly in US and Europe, the prevalence of obesity is also increasing in Japan. The rise in the incidence of obesity has prompted the WHO to recognize obesity as a significant disease. Two recently marketed anti-obesity agents, Xenical (Orlistat/Roche) and Meridia (Reductil/BASF) exhibit only modest efficacy (Orlistat) and have safety/side effect concerns (Orlistat-gastrointestinal and Meridia-hypertensive effects, respectively), that limit their use.
Thus, although there are a variety of anti-atherosclerosis, obesity and diabetes therapies, there is a continuing need and a continuing search in this field of art for alternative therapies.
Non-alcoholic fatty liver disease (NAFLD), and the more pathologic liver disorder, non-alcoholic steatohepatitis (NASH), develop from fat accumulation in the liver. Some degree of NAFLD affects up to one third of the general population. In particular insulin resistance, type 2 diabetes, obesity, hypertriglyceridemia, and female gender are independently associated with NAFLD. NAFLD is found in 30-100% of subjects with one or more metabolic abnormalities and is found in the majority of subject with type 2 diabetes. NAFLD is not only found in adults, but is also present in obese/diabetic children and adolescents. Patients with, or being treated for, human immunodeficiency virus are also at a much greater risk of developing NAFLD. Recent studies indicate that the progression of NAFLD to NASH can result in the development of fibrous tissue in the liver (fibrosis) in up to 40% of patients or cirrhosis in 5-10% of patients. Current treatments are limited, relying largely on exercise and weight loss.
Moreover, in patients with NAFLD and insulin resistance, de novo lipogenesis may contribute up to 25% of total liver lipid. It has been noted that patients with NAFLD have substantially increased mRNA levels of both ACC1 and ACC2, compared to control subjects.
Atherosclerosis, a disease of the arteries, is recognized to be the leading cause of death in the United States and Western Europe. The pathological sequence leading to atherosclerosis and occlusive heart disease is well known. The earliest stage in this sequence is the formation of “fatty streaks” in the carotid, coronary and cerebral arteries and in the aorta. These lesions are yellow in color due to the presence of lipid deposits found principally within smooth muscle cells and in macrophages of the intima layer of the arteries and aorta. Further, it is postulated that most of the cholesterol found within the fatty streaks, in turn, gives rise to development of the “fibrous plaque,” which consists of accumulated intimal smooth muscle cells laden with lipid and surrounded by extra-cellular lipid, collagen, elastin and proteoglycans. These cells plus matrix form a fibrous cap that covers a deeper deposit of cell debris and more extracellular lipid. The lipid is primarily free and esterified cholesterol. The fibrous plaque forms slowly, and is likely in time to become calcified and necrotic, advancing to a “complicated lesion,” which accounts for arterial occlusion and a tendency toward mural thrombosis and arterial muscle spasm that characterize advanced atherosclerosis. Hyperlipidemia has been established as a primary risk factor in causing cardiovascular disease (CVD) due to atherosclerosis. In recent years, leaders of the medical profession have placed renewed emphasis on lowering plasma cholesterol levels, and low density lipoprotein cholesterol in particular, as an essential step in prevention of CVD. The upper limits of “normal” are now known to be significantly lower than heretofore appreciated. As a result, large segments of Western populations are now realized to be at particularly high risk. Additional independent risk factors include glucose intolerance, left ventricular hypertrophy, hypertension, and being of the male sex. Cardiovascular disease is especially prevalent among diabetic subjects, at least in part because of the existence of multiple independent risk factors in this population. Successful treatment of hyperlipidemia in the general population, and in diabetic subjects in particular, is therefore of exceptional medical importance.
More than half a century ago it was first demonstrated that fatty acid synthesis in tumor tissues occurs at a very high rates. A number of studies have demonstrated that in tumor cells almost all fatty acids derive from de novo synthesis despite adequate nutritional supply. In addition, tumors overexpressing fatty acid synthase (FAS), the enzyme responsible for de novo synthesis of fatty acids, display aggressive biologic behavior compared to those tumors with normal FAS levels, suggesting that FAS overexpression confers a selective growth advantage.